syscalls - gate to the linux kernel

https://events.opensuse.org/conference/oSC16/program/proposal/840

Transcript

1. Syscalls
   openSUSE Conference 2016
   By Bernhard M. Wiedemann
   bwiedemann@SUSE
   

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2. © ScotXW / Wikipedia License: CC BY-SA 3.0
   

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3. An Example
   #include
   int main()
   {
   printf("Hello World!\n");
   return 0;
   }
   

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4. In the glibc
   ●
   /usr/include/asm-generic/unistd.h:
   #define __NR_write 64
   __SYSCALL(__NR_write, sys_write)
   

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5. vDSO
   > ldd hello1
   linux-vdso.so.1 (0x00007fffa07e2000)
   libc.so.6 => /lib64/libc.so.6 (0x00007fb892c48000)
   /lib64/ld-linux-x86-64.so.2 (0x00007fb893018000)
   

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6. On the Kernel-Side
   ●
   http://lxr.free-electrons.com/source/fs/open.c#L1023
   SYSCALL_DEFINE3(open,
   const char __user *, filename, int, flags, umode_t, mode)
   {
   if (force_o_largefile())
   flags |= O_LARGEFILE;
   return do_sys_open(AT_FDCWD, filename, flags, mode);
   }
   

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7. bootcd:/ # echo newuser::500:500:TestUser:/home/newuser:/bin/bash >>/etc/passwd
   bash: /etc/passwd: Read­only file system
   bootcd:/ # insmod translucency
   Using /lib/modules/2.4.18­4GB/kernel/fs/translucency.o
   bootcd:/ # echo "/ ­> /var/tmp" > /proc/sys/translucency/0
   bootcd:/ # echo newuser::500:500:TestUser:/home/newuser:/bin/bash >>/etc/passwd
   bootcd:/ # tail ­3 /etc/passwd
   perforce:x:68:60:perfoce admin:/var/lib/perforce:/bin/false
   nobody:x:65534:65534:nobody:/var/lib/nobody:/bin/bash
   newuser::500:500:TestUser:/home/newuser:/bin/bash
   bootcd:/ # echo > /proc/sys/translucency/0
   bootcd:/ # rmmod translucency
   bootcd:/ # tail ­3 /etc/passwd
   pop:x:67:100:pop admin:/var/lib/pop:/bin/false
   perforce:x:68:60:perfoce admin:/var/lib/perforce:/bin/false
   nobody:x:65534:65534:nobody:/var/lib/nobody:/bin/bash
   bootcd:/ # ls ­l /var/tmp/etc/
   total 4
   drwxr­xr­x 2 root root 0 May 11 15:51 .
   drwxrwxrwt 6 root root 0 May 11 15:51 ..
   ­rw­r­­r­­ 1 root root 2852 May 11 15:51 passwd
   bootcd:/ #
   What else you can do with
   syscalls
   

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8. Thanks
   ●
   Questions?
   

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